Solenoid linear actuators are well known for use in controlled linear displacement is of apparatus such as poppet valve stems. An exemplary use is in an exhaust gas recirculation valve (EGR valve) on an internal combustion engine.
In general construction terms, an insulated wire is wound in a plurality of turns (coil or windings) around a hollow cylindrical bobbin. A primary pole piece (PPP) includes a flange that extends across a first axial face of the bobbin and coil and also includes a first inner skirt extending axially into the bobbin hollow. A secondary pole piece (SPP) extends across a second and opposite axial face of the bobbin and coil and also includes a second skirt extending axially into the hollow toward the first skirt. The first and second skirts are separated by a calculated annular air gap, and the magnetic path between the PPP and the SPP typically is completed by a ferromagnetic housing around the windings. A ferromagnetic armature is slidably disposed within the cylindrical bobbin.
When current is passed through the windings, a toroidal magnetic field is created having axial field lines coincident with the axis of the bobbin and windings, which cause the armature to be urged axially of the windings. In a poppet valve application, the valve stem is attached to the armature and thus the valve head is urged into and out of closing contact with a valve seat in a valve body.
It is known in the prior art to conically taper the PPP inner skirt to provide a near-sharp terminal edge on the skirt at the air gap, the inner surface of the skirt being cylindrical and the outer surface being conical. The PPP acts as a flux carrier, and the flux path jumps the gap at the shaped edge. The conically tapered skirt both intensifies the flux density at the air gap and aids mechanically in guiding the armature as it slides in a thin stainless steel liner within the bobbin and windings.
In the prior art, an inexpensive means for forming solenoid components is powdered metal (PM) forming. Unfortunately, it has been found to be extremely difficult to form directly by PM a PPP conical skirt having a properly shaped edge. Further, density is sacrificed in taper wall design, and wall thickness is difficult to control. Near-sharp cones cannot be produced via PM process without secondary machining of an oversize PPP PM blank, resulting in increased manufacturing cost in time, apparatus, materials, and cleanup of machining waste. Further, the edge of a prior art near-sharp cone is easily damaged in handling before and during assembly into a solenoid actuator.
What is needed in the art is an improved solenoid primary pole piece that is net-size formed of powdered metal and thus requires no post-molding machining.
What is further needed in the art is an improved solenoid primary pole piece that is more rugged than a prior art conical pole piece and is not readily damaged during assembly of a solenoid actuator.
It is a primary object of the invention to reduce manufacturing cost and increase assembly reliability of a solenoid actuator.